The literal meaning of panentheism is that all things are in
God. I.e., unlike pantheism, the world does not exhaust the totality of
God. Today the term is used especially by process theologians, who
emphasize the idea that there is an essential relationship between God
and the world, & that one cannot talk about God in complete independence
from the world, though the world is not simply equated with God.
However, Acts 17:28 suggests some idea of panentheism in a non-process
context.
Panentheism is not the same as the traditional idea of
omnipresence, though there are relationships between them. If "God is
everywhere" but the world does not limit God's presence, then some idea
of panentheism is called for - though again, not necessarily of the
process variety.
The word is also used sometimes in an imprecise way to mean the
belief that God is in all things.